Navigating images using image based geometric alignment and object based controls

ABSTRACT

Over the past few years there has been a dramatic proliferation of digital cameras, and it has become increasingly easy to share large numbers of photographs with many other people. These trends have contributed to the availability of large databases of photographs. Effectively organizing, browsing, and visualizing such .seas. of images, as well as finding a particular image, can be difficult tasks. In this paper, we demonstrate that knowledge of where images were taken and where they were pointed makes it possible to visualize large sets of photographs in powerful, intuitive new ways. We present and evaluate a set of novel tools that use location and orientation information, derived semi-automatically using structure from motion, to enhance the experience of exploring such large collections of images.

CROSS-REFERENCE TO RELATED APPLICATIONS

The instant application is a continuation application of, and claimspriority to, U.S. patent application Ser. No. 13/418,774, filed Mar. 13,2012, which issued Jun. 11, 2013 with U.S. Pat No. 8,463,071. U.S.patent application Ser. No. 13/418,774 is a divisional application ofU.S. patent application Ser. No. 11/493,436, filed Jul. 25, 2006, whichissued Apr. 17, 2012, with U.S. Pat. No. 8,160,400. U.S. patentapplication Ser. No. 11/493,436 claims priority to U.S. provisionalpatent application No. 60/737,908, filed Nov. 17, 2005. U.S. patentapplication Ser. No. 13/418,774 is incorporated herein in its entirety.U.S. patent application Ser. No. 11/493,436 is incorporated herein inits entirety. U.S. Pat. No. 8,160,400 is incorporated herein in itsentirety. U.S. provisional patent application No. 60/737,908 isincorporated herein in its entirety.

GOVERNMENT RIGHTS

The subject matter disclosed herein was made with government supportusing award No. IIS-0413198and DGE0203031awarded by the National ScienceFoundation. The Government has certain rights in the disclosed subjectmatter.

BACKGROUND

Digital cameras have become commonplace, and advances in technology havemade it easy for a single person to take thousands of photographs andstore all of them on a hard drive. At the same time, it has become mucheasier to share photographs with others, whether by posting them on apersonal web site, or making them available to a community ofenthusiasts using a photo-sharing service. As a result, anyone can haveaccess to millions of photographs through the Internet. Sorting throughand browsing such huge numbers of photographs, however, is a challenge.At the same time, large collections of photographs, whether belonging toa single person, or contributed by thousands of people, create excitingopportunities for enhancing the browsing experience by gatheringinformation across multiple photographs. Some photo-sharing services,such as FLICKR®, available at www.flickr.com, allow users to tag photoswith keywords, and provide a text search interface for finding photos.However, tags alone often lack the level of specificity required forfine-grained searches, and can rarely be used to organize the results ofa search effectively. For example, searching for “Notre Dame” in FLICKR®results in a list of thousands of photographs, sorted either by date orby other users' interest in each photo. Within this list, photographs ofboth the inside and the outside of Notre Dame cathedral in Paris areinterspersed with photographs taken in and around the University ofNotre Dame.

Finding a photograph showing a particular object, for instance, the doorof the cathedral, amounts to inspecting each image in the list.Searching for both “Notre Dame” and “door” limits the number of imagesto a manageable number, but almost certainly excludes relevant imageswhose owners simply omitted the tag “door.”

The computer vision community has conducted work on recovering cameraparameters and scene geometry from sets of images. The work of Brown andLowe [2005] and of Schaffalitzky and Zisserman [2002] involvesapplication of automatic structure from motion to unordered data sets. Amore specific line of research focuses on reconstructing architecturefrom multiple photographs, using semi-automatic or fully automaticmethods. The semi-automatic Facade system of Debevec, et al. [1996] hasbeen used to create compelling fly-throughs of architectural scenes fromphotographs. Werner and Zisserman [2002] developed an automatic systemfor reconstructing architecture, but was only demonstrated on small setsof photographs.

Techniques have been developed for visualizing or searching throughlarge sets of images based on a measure of image similarity (histogramdistances such as the Earth Mover's Distance [Rubner et al. 1998] areoften used). A similarity score gives a basis for performing tasks suchas creating spatial layouts of sets of images or finding images that aresimilar to a given image, but often the score is computed in a way thatis agnostic to the objects in the scene (for instance, the score mightjust compare the distributions of colors in two objects). Therefore,these methods are most suitable for organizing images of classes ofobjects, such as mountains or sunsets.

Finally, several tools have been developed for organizing large sets ofimages contributed by a community of photographers. For example, theWorld-Wide Media eXchange (WWMX) is one such tool. WWMX allows users tocontribute photographs and provide geo-location information by using aGPS receiver or dragging and dropping photos onto a map. However, thelocation information may not be extremely accurate, and the browsinginterface of WWMX is limited to an overhead map view. Otherphoto-sharing tools, such as FLICKR®, do not explicitly use locationinformation to organize users' photographs, although FLICKR® supportstools such as “Mappr” for annotating photos with location, and it ispossible to link images in FLICKR® to external mapping tools such asGOOGLE® Earth.

Finally, the following references are relevant to the description of theinvention.

ARYA, S., MOUNT, D. M., NETANYAHU, N. S., SILVERMAN, R., AND WU, A. Y.1998. An optimal algorithm for approximate nearest neighbor searchingfixed dimensions. Journal of the ACM 45, 6, 891-923.

BROWN, M., AND LOWE, D. G. 2005. Unsupervised 3D object recognition andreconstruction in unordered datasets. In International Conference on 3DImaging and Modeling.

CANNY, J. 1986. A computational approach to edge detection. IEEE Trans.Pattern Anal. Mach. Intell. 8, 6, 679-698.

DEBEVEC, P. E., TAYLOR, C. J., AND MALIK, J. 1996. Modeling andrendering architecture from photographs: a hybrid geometry-andimage-based approach. In SIGGRAPH '96: Proceedings of the 23rd annualconference on Computer graphics and interactive techniques, ACM Press,New York, N.Y., USA, 11-20.

Flickr. http://www.flickr.com.

HARTLEY, R. I., AND ZISSERMAN, A. 2004. Multiple View Geometry inComputer Vision, second ed. Cambridge University Press, ISBN:0521540518.

JOHANSSON, B., AND CIPOLLA, R. 2002. A system for automaticpose-estimation from a single image in a city scene. In IASTED Int. ConfSignal Processing, Pattern Recognition and Applications.

LOURAKIS, M. I., AND ARGYROS, A. A. 2004. The design and implementationof a generic sparse bundle adjustment software package based on thelevenberg-marquardt algorithm. Tech. Rep. 340, Institute of ComputerScience—FORTH, Heraklion, Crete, Greece, Aug. Available fromhttp://www.ics.forth.ge/˜lourakis/sba.

MIKOLAJCZYK, K., AND SCHMID, C. 2005. A performance evaluation of localdescriptors. IEEE Transactions on Pattern Analysis & MachineIntelligence 27, 10, 1615-1630.

RUBNER, Y., TOMASI, C., AND GUIBAS, L. J. 1998. A metric fordistributions with applications to image databases. In Int'l Conf. onComputer Vision (ICCV), 59-66.

SCHAFFALITZKY, F., AND ZISSERMAN, A. 2002. Multi-view matching forn-ordered image sets, or “How do I organize my holiday snaps?” InProceedings of the 7^(th) European Conference on Computer Vision,Copenhagen, Denmark, vol. 1, 414-431.

SUTHERLAND, I. E. 1964. Sketchpad: a man-machine graphical communicationsystem. In DAC ′64: Proceedings of the SHARE design automation workshop,ACM Press, New York, N.Y., USA, 6.329-6.346.

SZELISKI, R. 2005. Image alignment and stitching: A tutorial. Tech. Rep.MSR-TR-2004-92, Microsoft Research.

WERNER, T., AND ZISSERMAN, A. 2002. New techniques for automatedarchitecture reconstruction from photographs. In Proceedings of the 7thEuropean Conference on Computer Vision, Copenhagen, Denmark, vol. 2,541-555.

WWMX. World-Wide Media eXchange. http://www.wwmx.org.

YEH, T., TOLLMAR, K., AND DARRELL, T. 2004. Searching the web withmobile images for location recognition. In CVPR (2), 76-81.

SUMMARY

Many collections of photos can be organized, browsed, and visualizedmore effectively using more fine-grained knowledge of location andorientation. As a simple example, if, in addition to knowing simply thata photograph was taken at a place called “Notre Dame” we know thelatitude and longitude the photographer was standing along with theprecise direction he was facing, then an image of the door to Notre Damecathedral can be found more easily by displaying search hits on a mapinterface, and searching only among the images that appear in front ofthe cathedral door.

As well as improving existing search tools, knowing where a photo wastaken makes many other browsing modes possible. For instance, relatingimages by proximity makes it possible to find images that were takennearby, or to the left of, or north of, a selected image, or to findimages that contain a close-up of a part of another image. Withknowledge of location and orientation, it is easier to generate morphsbetween similar photographs, which can make the relationship betweendifferent images more explicit, and a browsing experience morecompelling. Location and orientation information can be combined withother metadata, such as date, time, photographer, and knowledge ofcorrespondence between images, to create other interestingvisualizations, such as an animation of a building through time. Withadditional knowledge of the geometry of the scene, location informationalso allows tags associated with parts of one photograph to betransferred to other similar photographs. This ability can improve textsearches, and the access to additional information for each photo canfurther enhance the browsing experience.

These browsing tools can be applied to a single user's photo collection,a collection of photos taken for a special purpose (such as creating avirtual tour of a museum), or a database containing photos taken by manydifferent people.

We also describe herein new tools and interfaces for visualizing andexploring sets of images based on knowledge of three-dimensional (3D)location and orientation information, and image correspondence. Wepresent semi-automatic techniques for determining the relative andabsolute locations and orientations of the photos in a large collection.We present an interactive image exploration system. These and otheraspects and embodiments of the invention are described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The systems and methods for navigating images using image basedgeometric alignment and object based controls in accordance with thepresent invention are further described with reference to theaccompanying drawings in which:

FIG. 1 illustrates a general operating environment for the invention.

FIG. 2 illustrates an exemplary method for determining relative andabsolute location information for a plurality of digital photographs.

FIG. 3 illustrates an exemplary overhead map interface that may be used,in one embodiment, for registering new photographs in a photo set, andin another embodiment, for browsing photos by selecting from the map acamera location that is desired for viewing.

FIG. 4 illustrates a plurality of user interface features in anexemplary “free-flight” browsing mode, in which a user can move avirtual camera in a representation of a 3D geometry and select desiredcamera positions for viewing a corresponding digital photo.

FIG. 5 illustrates a plurality of user interface features in anexemplary “image-based” browsing mode, in which a user may see a firstphotograph in a main location in the interface and also have access to aplurality of selectable alternate images that may have image contentrelated to the first photograph.

FIG. 6 illustrates another exemplary embodiment of a “free flight”browsing mode such as presented in FIG. 4.

FIG. 7 illustrates another exemplary embodiment of a “image-based”browsing mode such as presented in FIG. 5.

FIG. 8 illustrates a sample triangulation of a set of sparse 3D pointsand line segments, used for morphing. The triangulation is superimposedon the image that observed the 3D features.

FIG. 9 illustrates an exemplary information and search pane comprising aplurality of search tools that may be incorporated into embodiments ofthe invention.

FIG. 10 illustrates a plurality of user interface features in anexemplary “object-based” browsing mode, in which a user can select anobject and find other images also containing the object, and moreovermay sort images by which have “best” views of the selected object.

FIG. 11 illustrates a plurality of user interface features in anexemplary “object-based” browsing mode, in which a user selected anobject in FIG. 10, and was presented with a best view of the object inFIG. 11 along with a plurality of other views of the object in 1102,which may be ordered according to which have best views of object 1000.

FIG. 12 illustrates an exemplary digital photograph 1200 which may bepresented in various user interfaces presented herein, and metadatarelating to image attributes, tags, and annotations to portions of thephotograph which may also be presented along with the photograph 1200.

FIG. 13 illustrates images from a Notre Dame data set showing thecathedral from approximately the same viewpoint, but at different times.The various images 1301-1304 may be presented in a stabilized slideshow. The annotation of the rose window 1310 has been transferred fromimage 1301 to the other three images 1302-1304.

DETAILED DESCRIPTION

Certain specific details are set forth in the following description andfigures to provide a thorough understanding of various embodiments ofthe invention. Certain well-known details often associated withcomputing and software technology are not set forth in the followingdisclosure, however, to avoid unnecessarily obscuring the variousembodiments of the invention. Further, those of ordinary skill in therelevant art will understand that they can practice other embodiments ofthe invention without one or more of the details described below.Finally, while various methods are described with reference to steps andsequences in the following disclosure, the description as such is forproviding a clear implementation of embodiments of the invention, andthe steps and sequences of steps should not be taken as required topractice this invention.

The systems and methods for navigating images using image basedgeometric alignment and object based controls described herein have beenapplied to a variety of data sets comprising images from variouslocations. Thus, the various techniques and figures discussed may makeoccasional reference to the tested data sets. Tested data sets include,for example, a set of photographs of the Old Town Square in Prague,Czech Republic, a set of photographs taken along the Great Wall ofChina, a set of photos resulting from an internet search for “notredameAND paris,” a set of photos resulting from an internet search for“halfdome AND Yosemite,” a set of photos resulting from an internetsearch for “trevi AND rome,” and a set of photos resulting from aninternet search for “trafalgarsquare.”

General Operating Environment

FIG. 1 presents a general operating environment for aspects of theinvention. In general, computer hardware and software such as thatdepicted in FIG. 2 may be arranged in any configuration and using thefull extent of presently available or later developed computing andnetworking technologies. In one configuration, a server 100 may beconnected to a network 105 such as the internet. The server 100 mayreceive and respond to requests from client computers such as 110 thatare also coupled to the network 105. Server 100 may be equipped with orotherwise coupled to a database or data store 101 containing images suchas digital photographs, as well as metadata or other useful informationthat can be used to categorize and process the images. Server 100 mayalso be equipped with or otherwise coupled to image processing logic 102for carrying out various processing tasks as discussed herein.

Thus, in one arrangement, a client 110 may request data from a server100 via network 105. The request may be in the form of a browser requestfor a web page, or by other means as will be appreciated by those ofskill in the art. The server 100 may provide the requested information,which can be used by the client 110 to present a user interface ondisplay 120. A user can interact with the user interface by activatingselectable objects, areas, icons, tools and the like using a selectiondevice 130 such as a mouse, trackball or touchpad. In connection withproviding such information, the server 100 may access database 101 forappropriate images and may apply image processing logic 102 asnecessary. Certain image processing logic in 102 may also be appliedbefore and after the client request to properly prepare for and ifnecessary recover from satisfaction of the client 110 request. Inconnection with displaying the requested information, client 110 may, insome embodiments, access its own database 111 and image processing 112,for example when the client 110 and server each contain information tobe presented in a particular user interface on electronic display 120.In other embodiments, the client 110 may simply rely on the server 100to provide substantially all of the image processing functionsassociated with carrying out the invention.

In another arrangement, the client 110 may implement the systems andmethods of the invention without relying on server 100 or network 105.For example, client 110 may contain images in database 111, and mayapply image processing logic 112 to the images to produce a userinterface that can be presented to a user via display 120. Thus, whilethe invention can be performed over a network using client/server ordistributed architectures as are known in the art, it is not limited tosuch configurations and may also be implemented on a stand-alonecomputing device.

The description and figures presented herein can be understood asgenerally directed to hardware and software aspects of carrying outimage processing logic such as 102 and 112 that produces at least inpart a user interface that may be presented on an electronic display120. Many of the remaining figures, as will be appreciated, are directedto exemplary aspects of a user interface that may be presented on adisplay 120. Aspects of the invention comprise novel features of suchuser interfaces, as will be appreciated, and optionally also supportinglogic 112, 102 that produces such aspects of user interfaces or thatprocesses images such that they may be presented in a user interface asdisclosed herein.

Determining Geo-Location

In order to effectively use our browsing tools on a particular set ofimages, we need fairly accurate information about the location andorientation of the camera used to take each photograph in the set. Inaddition to these extrinsic parameters, it is useful to know theintrinsic parameters, such as the focal length, of each camera. How canthis information be derived? GPS is one way of determining position, andwhile it is not yet common for people to carry around GPS units, nor doall current GPS units have the accuracy we desire, a first solution isto equip digital cameras with GPS units so that location and orientationinformation can be gathered when a photograph is taken. As for theintrinsic parameters, many digital camera models embed the focal lengthwith which a photo was taken (as well as other information, such asexposure, date, and time) in the Exchangeable Image File Format (EXIF)tags of the image files. EXIF is the present standard for imagemetadata, but any image metadata may also be used. However, EXIF and/orother metadata values are not always accurate.

A second solution does not rely on the camera to provide accuratelocation information; instead, we can derive location using computervision techniques. Brown and Lowe 2005 provides useful background forthis discussion. We first detect feature points in each image, thenmatch feature points between pairs of images, keeping only geometricallyconsistent matches, and run an iterative, robust structure from motionprocedure to recover the intrinsic and extrinsic camera parameters.Because structure from motion only estimates the relative position ofeach camera, and we are also interested in absolute coordinates (e.g.,latitude and longitude), we use a novel interactive technique toregister the recovered cameras to an overhead map. A flowchart of theoverall process is shown in FIG. 2.

As can be observed in FIG. 2, a set of input images 200 can be processedthrough a variety steps as may be carried out by one or more computersoftware and hardware components, to ultimately produce informationregarding the absolute location of the input images (photographs), andthe 3D points within such images 212. Exemplary steps can includekeypoint detection 201, keypoint matching 202, estimating epipolargeometry and removing outliers 203, applying a structure from motionprocedure 204 that produces an output comprising the relative locationsof photographs and 3D points 210, and map registration 211.

The exemplary structure from motion procedure 204 may comprise choosinga pair of images I₁ and I₂ with a large number of matches and widebaseline 205, running bundle adjustment 206, choosing a remaining imageI_(k) with the most matches to existing points in the scene and addingimage I_(k) to the optimization 207, again running bundle adjustment asnecessary 208, adding well-conditioned points to the optimization 209.Additional images can be processed as necessary by returning to step206. After all images are processed, output 210 can be used in mapregistration 211 as described above. Various exemplary aspects of asystem such as that of FIG. 2 are discussed in greater detail in thebelow sections, entitled “keypoint detection and matching,” “structurefrom motion,” “interactive registration to overhead map,” “registeringnew photographs,” and “line segment reconstruction.”

Keypoint Detection and Matching

Detecting feature points in a plurality of images and matching featurepoints between two or more of said plurality of images may comprise thefollowing procedures for estimating image location. The first step is touse a keypoint detector, such as any of the various keypoint detectorsdescribed in Mikolajczyk and Schmid 2005. A keypoint detector detectskeypoints for each image. We then match keypoint descriptors betweeneach pair of images. This can be done, for example, using theapproximate nearest neighbors technique of Arya et al. 1998. Any otheracceleration technique could also be used, including but not limited tohashing or context-sensitive hashing. For each image pair with a largeenough number of matches, we estimate a fundamental matrix using, forexample Random Sampling Consensus (RANSAC), or any other robustestimation technique, and remove the matches that are outliers to therecovered fundamental matrix. After finding a set of putative,geometrically consistent matches, we organize the matches into a set oftracks, where a track is simply a set of mutually matching keypoints;each track ideally contains projections of the same 3D point.

If the keypoints in every image form the vertex set of a graph, andthere is an edge in the graph between each pair of matching keypoints,then every connected component of this graph comprises a track. However,the tracks associated with some connected components might beinconsistent; in particular, a track is inconsistent if it contains morethan one keypoint for the same image. We keep only the consistent trackscontaining at least two keypoints for the next phase of the locationestimation procedure. Note that this simple rejection of nominallyinconsistent tracks will not reject all physically inconsistent tracks(i.e., tracks that contain keypoints that are projections of different3D points).

Structure from Motion

Next, we wish to determine a plurality of relative locations of saidimages. This step can comprise recovering a set of camera parameters anda 3D location for each track. We make the common assumption that theintrinsic parameters of the camera have a single degree of freedom, thefocal length. The recovered parameters should be geometricallyconsistent, in that when each track is projected into each image thatobserves that track, the distance between the projected point and thedetected keypoint is low. Recovering the parameters can be formulated asa non-linear least squares problem. In particular, we have a set of ncameras, parameterized by position c_(i), orientation R_(i) (expressedusing three parameters as an iterative update to an initial rotationmatrix), and focal length f_(i); we group these parameters into a singleparameter vector θ_(i).

We also have a set of m tracks, parameterized by 3D location, p_(j). Ifwe assume that each camera observes each track, then we also have a setof 2D projections, q_(ij), where q_(ij) is the observed projection ofthe j-th track in the i-th camera. Let P(θ, p) be the projectionequation, i.e., the equation that maps 3D points to their 2D projectionson a camera with parameters θ. P first transforms p to the camera'scoordinate system, then performs the perspective division and multipliesby the focal length to transform the result to image coordinates:p′(θ, p)=R(p−c)P(θ, p)=[−fp′ _(x) /p′ _(z) −fp′ _(y) /p′ _(z)]^(T)

The quantity we wish to minimize is the sum of the reprojection errors:

$\sum\limits_{i = 1}^{n}{\sum\limits_{j = 1}^{m}{w_{ij}{{q_{ij} - {P\left( {\theta_{i},p_{j}} \right)}}}}}$

(In practice, not every track will be observed by every camera, sow_(ij) is used as an indicator variable; w_(ij)=1 if camera i observespoint j, and w_(ij)=0 otherwise).

This non-linear optimization problem can be solved with algorithms suchas Levenberg-Marquardt. Such algorithms are only guaranteed to findlocal minima, however, so it is important to provide them with a goodinitial estimate of the parameters. Rather than try to estimate theparameters of all cameras and tracks at once, then, we take anincremental approach in which we add in cameras one at a time withcareful initial parameter estimates.

In this incremental procedure, we start out by estimating the parametersof a single pair of cameras, and the locations of the tracks seen bythat pair. As a heuristic, we choose a pair of images that have a largenumber of matches, but whose matches cannot be well-modeled by ahomography (indicating that the displacement between the two cameras isrelatively large compared with the distance to the scene). Next, wechoose the camera that observes that largest number of tracks whoselocations have already been estimated, and add that camera into theoptimization. We initialize the new camera's extrinsic parameters usingthe Direct Linear Transform (DLT) technique described in Sutherland1964, although any of a variety of well-known camera calibrationtechniques could be used. We initialize the focal length to either theestimate obtained from the EXIF tags for that image (if one exists), orto the focal length estimated using DLT. We then rerun the optimizationto refine the parameters for each of the cameras.

Finally, we add tracks observed by the new camera into the optimization.A track is added if it is observed by at least one other camera whoseparameters have already been estimated, and only if that camera is farenough away from the new camera that we can get a well-conditionedestimate of the location of the track. We initialize the 3D location ofeach new track using triangulation. This procedure is repeated until nomore cameras can be added to the optimization.

As described so far, our optimization technique is not robust tooutliers, i.e., tracks that contain projections of multiple 3D points.We make it more robust with two modifications: first, when estimatingthe initial location and orientation of a camera, we apply the DLTtechnique in conjunction with RANSAC. Second, after every run of theoptimization, we reject as outliers tracks that contain at least onekeypoint with a high reprojection error. If any outliers are rejected,we rerun the optimization.

To solve the optimization at every iteration, we use the technique ofbundle adjustment, for example, the technique described in Hartley andZisserman 2004 is one workable option. In particular, the sparse bundleadjustment technique of Lourakis and Argyros 2004 is another workableoption that may be preferable in some circumstances. This techniquetakes advantage of the sparseness of the problem to find a minimum in areasonable amount of time and using a reasonable amount of memory evenfor large numbers of points and cameras.

Interactive Registration to Overhead Map

The final step of the location estimation process is to determine basedon an absolute location of at least one of said images a plurality ofabsolute locations of said images. This step can determine the absolutecoordinates, for example latitude and longitude, of each camera. Thestructure from motion procedure estimates relative image locations. Ifthe relative locations have been estimated perfectly in that step, thenthe relative locations are related to the absolute locations by asimilarity transform (global translation, rotation, and uniform scale).Our approach for determining the correct transformation is to have auser interactively align the recovered camera locations and 3D pointswith a satellite image or floor plan. First, the user must provide anappropriate image. Next, to assist the user in the alignment, weestimate the “up” direction in the image by fitting a plane to therecovered camera centers (this assumes that all the photographs weretaken from approximately the same height). The normal to this plane istaken to be the up vector; to resolve the ambiguity of whether thenormal points upwards or downwards, we assume that pictures are moreoften taken with the camera held right-side-up rather than up-side-down,and pick the direction that agrees with the orientation of the majorityof the images. An alternate technique for estimating the up vector isgiven in Szeliski 2005, and is also considered workable in embodimentsof the invention. The scene is then rendered superimposed on thealignment image, using an orthographic projection with the camerapositioned above the scene, pointed downward.

Next, the user can align the recovered geometry to the alignment imagein one of two ways. First, the user can manually rotate, translate, andscale the model until it is in agreement with the alignment image. Ifthe up vector was determined accurately, the user needs only to rotatethe model in 2D, rather than 3D. We have observed that it is oftenfairly easy to perform this alignment by matching the recovered pointsto features, such as building facades, visible in the image.

A screenshot showing a model recovered from the Prague data set that hasbeen manually aligned to a satellite image is shown in FIG. 3. In FIG.3, camera positions and orientations are represented as a plurality offrusta overlaid on top of an overhead map. Second, a user can also dragand drop a few images to their approximate locations on the map. Oursystem then computes the best similarity transform as each image isadded, then renders the transformed model. Once a camera has beenpositioned on the map, it can be dragged around to refine the fit. Whenthe user is satisfied with the alignment, the similarity transform isstored for future use.

It can sometimes happen that the recovered scene geometry is inaccurate,and no similarity transform will result in a good alignment with theprovided image. For instance, if a set of photographs of buildingfacades were taken while walking down a city street, the recoveredcamera positions may exhibit a slight curve instead of being perfectlystraight. Inaccuracies like these can sometimes be corrected by taking adenser set of photographs, or by correcting the images for radialdistortion. We also allow the user to interactively correct mistakes inthe geometry by first using the alignment tools to find an approximatesimilarity transform, then dragging one or more cameras to more accuratepositions and rerunning the optimization.

Registering New Photographs

Once the locations of an initial set of photographs has been estimated,new photographs can be registered in several ways. First, keypoints inthe new photo can be matched with a set of representative featuredescriptors computed for each track in the original set of images, andthe position of the new photo can then be estimated using our structurefrom motion procedure. Second, the user can drag the new photo onto themap at the approximate location of that photograph. The location of thatphoto is then estimated in the same way, but its keypoints are matchedwith a more limited number of tracks (those visible in nearby images).This makes the image snap into place more quickly.

Line Segment Reconstruction

Once we know the relative locations of each of the photographs, weoptionally reconstruct 3D line segments in the scene from the photos.Line segments can be useful for generating more convincing morphsbetween images and for creating non-photorealistic renderings, such asrenderings comprising constructions of 3D scenes using lines and points,sometimes referred to as “wireframe” renderings of architectural scenes.We reconstruct 3D line segments as follows:

-   -   1. Detect 2D line segments in each image.    -   2. Derive a set of candidate matches by comparing line segments        between pairs of nearby images.    -   3. Find sets of mutually consistent matches (line tracks) above        a certain size. Triangulate the matching 2D line segments to        obtain a 3D line segment.

The steps above will now be described in more detail. To detect 2D linesegments, we use Canny edge detection as described in Canny 1986. Anyedge detection technique may be selected. Edge detection is followed byan edge-linking step to form edge chains. Next, for each edge chain, wefit a line to the chain, and break the chain at the point of the chainfurthest from the line; we recursively apply this procedure until allchains approximate line segments. Finally, we remove chains smaller thana threshold, and store the two endpoints of the line segmentapproximation of each of the remaining chains. We will use S(I) todenote the set of line segments found in image I.

To match line segments between pairs of images, we first compute aneighbor relation on the set of images. Two images are defined asneighbors if they are pointing in approximately the same direction andthe number of 3D points both images observe is above a threshold. Inaddition, the images cannot be too close together (to ensure thattriangulating matching segments is well-conditioned). Next, for eachpair of matching images I and J, we consider each line segment s

S(I). Each line segment t

S(J) is labeled as a candidate match of s if t meets the following twoconditions:

-   -   1. The endpoints oft are not too far away from the epipolar        lines of the endpoints of s in image J.    -   2. The L₂ distance between a strip of intensity values around s        and a strip of intensity values around t is not too large.

The intensity values are sampled along epipolar lines, and each strip isnormalized for bias and gain before their L₂ distance is computed. Forline segments on the edge of a building or other structure, one side ofthe segment might be part of the background in one or both of theimages. Therefore, rather than using the strict L₂ distance, we dividethe intensity strips into two halves on either side of the line segment,compute the L₂ distance between the two halves, and keep the minimum ofthe two distances. After computing candidate matches between pairs ofneighbors, we consider connected components of candidate matches, as wedid with keypoints in the “Keypoint Detection and Matching” section,above. Unlike with the keypoint matching, however, we now know whereeach photograph was taken, so we can check connected components forphysical consistency. For each connected component of matches with asubset of consistent line segments of size at least four, we triangulatethe consistent matches and store the resulting 3D line segment.

Photo Browsing User Interface

Once a set of photographs of a scene has been registered as described inthe above sections or via any processes or procedures that yield thesame or similar result, the user can browse the photographs with a photobrowsing user interface. In the following sections, we describe aspectsof an exemplary photo browsing user interface. Aspects of the userinterface may be mixed and matched in a variety of ways, and can beusefully grouped into separate and distinct browsing modes. To activatefeatures of the user interface or switch between different browsingmodes, a user may select an appropriate button in the user interface, ormay select an item from a menu provided in the user interface. Thefollowing sections are thus directed to aspects of an exemplaryinterface and browsing modes that may be used to usefully group suchaspects.

User Interface Layout

FIG. 4 shows a screen shot from an exemplary photo exploration userinterface. FIG. 4, FIG. 5, and the other user interface illustrationscan be considered to be user interfaces displayed upon electronicallygenerated interactive displays. The components of the user interfacepresented in FIG. 4 are, first, a representation of a three-dimensional(3D) geometry from a first position and first orientation 400. Here, therepresentation of a 3D geometry is depicted in a main location in theuser interface while other elements of the user interface, e.g. 401, areoverlaid and to the sides of the main location. 3D geometry 400 shows arepresentation of the world as seen from a virtual camera controlled bythe user—the 3D geometry is configured to be navigated by a user. In theillustrated embodiment, this representation 400 is not meant to show aphotorealistic view of the scene, but rather to display camera positionsassociated with digital photographs in spatial context and give a senseof the geometry of the true scene. In one embodiment, the 3D geometry400 may comprise at least one translucent projection of a digitalphotograph onto said 3D geometry. In FIG. 4, the translucent projectionsare blended in with the line-drawn building geometry to provide shadingand color aspects of the representation 400. FIG. 6 provides a usefulexample of another embodiment of a translucent projection of a digitalphotograph 601.

In FIG. 4, the at least one translucent projection of a digitalphotograph onto 3D geometry 400 provides a more attractivevisualization. 3D geometry 400 may also be rendered using a washed-outcoloring to give an impression of scene appearance and geometry, but maybe abstract enough to be forgiving of the lack of detailed geometry.

In one embodiment, a translucent projection of a digital photograph on arepresentation such as 400 may be accomplished as follows: For eachcamera C_(j), embodiments may first robustly fit a plane toPoints(C_(j)) using RANSAC. If the number of inliers to the recoveredplane is at least 20% of the size of Points(C_(j)), embodiments may thenfit a robust bounding box, Rectangle(C_(j)), to the inliers in theplane. To render the scene, the embodiment may project a blurred,semi-transparent version of each image I_(j) onto Rectangle(C_(j)) anduse alpha blending to combine the results. In parts of the scenerepresented with a sparse number of points, one exemplary system mayfall back to point and line rendering.

FIG. 4 illustrates a plurality of drawn points and lines that form the3D geometry 400 of, in this case, streets, buildings, and the like. Therecovered points and lines are used to depict the scene 400 itself. Inone embodiment, the points may be rendered with their acquired color andthe lines may be drawn with thick black borders, to achieve a line-drawnlook. The user can be supplied with one or more user interface toolsthat control, for example, whether or not the points and lines aredrawn, the size of the points, and the thickness of the lines. FIG. 4also illustrates a map view 401 of the location associated with 3Dgeometry 400.

FIG. 4 illustrates an example of rendering using projected translucentimage projections overlaid with line segments. Here, the translucentprojections give the underlying shading and possibly colors, on top ofwhich points and lines may be rendered to complete the 3D geometry. InFIG. 4, camera positions and orientations are rendered within 3Dgeometry 400 as selectable frusta, e.g. frustum 401. Frustum 401 thusrepresents a selectable camera position associated with a digitalphotograph of the location depicted by 3D geometry 400. If a userselects frustum 401, the representation 400 can change to depict anenlarged view of the associated digital photograph, e.g., a view such as500 in FIG. 5.

The view 500 presented may be the back face of the selected camerafrustum 401. In one embodiment, the back face of the frustum 401 can betexture-mapped with an opaque, full-resolution version of the associatedphotograph, so that the user can see it in detail. The back faces ofother cameras' frusta can be texture-mapped with a low-resolution,semi-transparent thumbnail of the photos if the frustum is visible andnear the user's current position in the 3D geometry. In anotherembodiment, they may be rendered with a translucent white color.

FIG. 5 illustrates an enlarged view of a digital photograph 500 in amain location in a user interface. Three exemplary overlay panes areillustrated in FIG. 5. An information and search pane 502 on the left, athumbnail pane 503 along the bottom, and a map pane 501 in theupper-right corner.

In one embodiment, the information pane 502 appears when the user isvisiting a photograph 500. This pane 502 may display information aboutthe photo 500 in the main location, including for example its name, thename of the photographer, and the date and time when it was taken. Inaddition, the information pane 502 may contain search tools forsearching for other photographs with certain geometric relations to thecurrent photo 500.

The thumbnail pane 503 shows a plurality of selectable digitalphotographs of the location depicted by the 3D geometry 400. Thethumbnail pane 503 may show, for example, the results of searchoperations. The thumbnail pane 503 may be arranged, for example, as afilmstrip of selectable thumbnails as depicted in FIG. 5.

When the user is visiting a camera C_(curr), i.e., when the user isviewing a first photograph in main location 500, and the user selects athumbnail, e.g. by clicking or mouse-over, the corresponding image I_(j)can be projected onto a plane the main view 500 to give the user an ideaof the content of that image and how it is situated in space. In oneembodiment, we precompute projection planes, CommonPlane(C_(j); C_(k)),for each pair C_(j); C_(k) of cameras, by robustly fitting a plane toPoints(C_(j)) U Points(C_(k)). The thumbnail panel 503 may also havecontrols for sorting the current thumbnails by date and time, andviewing the photographs as a slideshow. Finally, the map pane 501displays an overhead view of scene that tracks the user's movement.

FIGS. 6 and 7 illustrate another exemplary embodiment of user interfacespresented in FIGS. 4 and 5. In FIG. 6, a representation of 3D geometry600 is presented in which a user can navigate around and viewtranslucent projections of photographs such as 601. FIG. 6 thus providesan exemplary free-flight navigation browsing mode, rendered as a wireframe with photographs shown as frusta floating in space. Here, thetranslucent projections are depicted on the back panes of frusta withinthe representation 600. The user may navigate the representation 600using, for example, arrow keys on the keyboard, mouse or joystickmovement. A camera position associated with a digital photograph may beselected by for example, clicking on a frustum and/or translucentprojection. Such action may take the user to a view such as that of FIG.7.

The free-flight navigation mode allows the user to move freely aroundthe location that is depicted by the 3D geometry 600. The goal of thismode is not to render a photorealistic view of the world at all times,but to let the user explore a set of photographs in a natural way, as ifthey were walking around a museum, pausing at interesting exhibits.

Therefore, the emphasis of this mode is on leading users to photographsthat they might be interested in. While the overhead map view can beviewed at many scales, the free-flight navigation mode is meant toexplore sets of photographs on scales comparable to that of a cityblock, town square, or single building.

In this mode, the user controls a virtual camera floating in space amongthe photographs. As with the overhead map view, the cameras may berendered as frusta, for example, and a semi-transparent thumbnail may betexture mapped on the “back face” of each frustum. Depending on the typeof scene, the scene itself can be rendered in several different ways. Ifa 3D model of the scene is available, then the model can be used as astand-in for the scene. For urban scenes, buildings can be rendered in“wireframe” mode using 3D line segments, and the ground can be renderedas a texture mapped satellite image or as a road map. For naturalscenes, the ground can be displayed as a height field using topographicdata and painted with an appropriate texture.

One set of navigation controls available to the user in this mode can bebasic 3D motions (left, right, forward, backwards, up, down, andcontrols for pitch and yaw), which allow for free motion through thescene. Because the emphasis is on visiting images, the user is notifiedwhen they are near a photograph by the appearance of a thumbnail on theborder of the window. The placement of the thumbnail indicates thedirection the user should go to view that photograph. In addition, theuser can allow photographs to exert a “pull” on the virtual camera, sothat the camera is automatically drawn towards a photograph that it isapproaching. At any time, the user can select a photograph by clickingon a frustum or on one of the thumbnails on the border of the window.Once a photograph is selected, the virtual camera may be smoothly movedto the position and orientation where the selected photograph was taken,and the frustum may be texture-mapped with an opaque full-resolutionimage.

In FIG. 7, a photograph has been selected, and related views aredisplayed on the boundary. A photograph is depicted in a main location700, and a plurality of selectable alternate photographs such as 701 arealso depicted. The user may go from a view with a first photo in 700 toa view with an alternate photo 701 in 700 by selecting such alternatephoto 701. The transition to such alternate photo may be made smoothlyusing one of the exemplary transition techniques provided herein.

Transitions Between Photographs

In one embodiment, a user interface such as provided in FIG. 5 may beconfigured to generate smooth transitions when the user moves betweenphotographs. Information we infer about the photographs such aslocation, orientation, and sparse correspondence allows us to use cameramotion and view interpolation to make transitions more visuallycompelling and to emphasize the spatial relationships between thephotographs. In other words, the system can be configured to display asequence of transitional views of the location depicted by the 3Dgeometry 400 prior to transitioning from displaying a first photographin 500 to displaying a selected digital photograph from 503. Thesequence of transitional views of said 3D geometry 400 can be generatedby projecting two images onto a common plane and cross-fading betweenthe projected images, as described in greater detail below.

Camera Motion

A variety of 3D games and exploration tools, such as GOOGLE® Earth, usesmooth camera motion, rather than sudden cuts, to make transitions moreappealing and intuitive. Smooth camera paths may also be used inembodiments of a user interface that implements other aspects of theinvention. Such embodiments may also tailor the implementation of smoothtransitions in several ways.

When the virtual camera is moved from one photograph to another, bydefault the camera center may be linearly interpolated between the startand end camera locations and the camera orientation can be linearlyinterpolated between unit quaternions representing the start and endorientations. The field of view of the virtual camera may also belinearly interpolated so that when it reaches its destination, thedestination image will fill as much of the screen as possible. Thecamera path timing can be non-uniform to ease in and out of thetransition. We also fade out all the camera frusta before starting themotion, to avoid flickering caused by the frusta rapidly moving throughthe view.

If the camera moves as the result of an object selection, the transitioncan be implemented differently. Before the camera starts moving, it canorient itself to point at the mean of the selected points. The cameracan remain pointed at the mean as it moves, so that the selected objectstays fixed in the view. This helps keep the object from undergoinglarge distracting motions during the transition. The end orientation andfocal length can be computed so that the selected object is centered andfills the screen.

View Interpolation

During camera transitions, some embodiments may be configured to displayin-between images. Two exemplary techniques for morphing between thestart and destination photographs are provided below: triangulating thepoint cloud and using planar impostors. Morphing may be conducted in avariety of contexts. For example, morphing may be conducted to provide asequence of transitional views when one of the thumbnails in FIG. 5 orFIG. 7 is selected. Morphing may also be conducted when a region withina photo is selected as will be described in greater detail below.

Triangulated Morphs

In one embodiment, to create a triangulated morph between two camerasC_(j) and C_(k), we first compute a 2D Delaunay triangulation for imageI_(j) using the projections of Points(C_(j)) into I_(j). The projectionsof Lines(C_(j)) into I_(j) are imposed as edge constraints on thetriangulation. The resulting Delaunay triangulation may not cover theentire image, so we overlay a grid onto the image and add each gridpoint not contained in the original triangulation. Each added grid pointis associated with a “virtual” 3D point on a plane approximating thegeometry of the points seen by both C_(j) and C_(k). The connectivity ofthe triangulation is then used to create a 3D mesh from Points(C_(j))and the endpoints of Lines(C_(j)). Finally, we texture map the mesh byprojecting image I_(j) onto each triangle. A Delaunay triangulation isalso computed for Ck and texture mapped in the same way.

To render an in-between view, each mesh can be rendered from the newviewpoint and blend the two rendered images in proportion to thedistance from the in-between camera to the two endpoints. While thistechnique does not use completely accurate geometry, the meshes areoften sufficient to give a good sense of the 3D geometry of the scene.However, missing geometry and outlying points can sometimes causedistracting artifacts, as demonstrated in the accompanying video.

In another embodiment, to provide more context for the relationshipbetween the current and next photos, we use view interpolation for thein-betweens. To render an in-between view, we use the recovered 3Dpoints (and 3D line segments, if available), as a geometric proxies forthe scene. First, for each of the two views, we compute a Delaunaytriangulation of the projection of each of the 3D points observed bythat view. If line segments have been detected, the projections of theendpoints of each line segment visible in that view are added to thetriangulation, and the edge between the endpoints is added as aconstraint. The resulting Delaunay triangulation may not cover theentire image, so we add additional points to the triangulation on a gridthe size of the image. Each grid point that lies outside of the initialDelaunay triangulation is added to the triangulation, and an associated3D point is added to the 3D point cloud. We position the 3D point eitheron the ground plane or the plane at infinity, depending on whether a rayfrom the image center through the grid point intersects the ground planein front of the image. A visualization of one of the resulting Delaunaytriangulations is shown in FIG. 8.

FIG. 8 provides a sample Delaunay triangulation of a set of sparse 3Dpoints and line segments, used for morphing. The triangulation issuperimposed on the image that observed the 3D features. Note thatoutside the extent of the observed features, the points in thetriangulation form a grid; these points were added so that the imagewould be fully covered.

We use the final Delaunay triangulation to introduce connectivity toeach of the two 3D point sets. We then texture map the two meshes byprojecting the appropriate image onto each mesh and render the twotexture-mapped meshes from the in-between viewpoint, blending them inproportion to the distance from the in-between camera to the twoendpoints.

Planar Morphs

To create a morph between cameras C_(j) and C_(k) using a planarimpostor, an exemplary embodiment may, for example, project the twoimages I_(j) and I_(k) onto CommonPlane(C_(j); C_(k)) and cross-fadebetween the projected images as the camera moves from C_(j) to C_(k).The resulting in-betweens are not as faithful to the underlying geometryas the triangulated morphs, tending to stabilize only a dominant planein the scene, but the resulting artifacts are usually lessobjectionable, perhaps because we are used to seeing distortions causedby viewing planes from different angles. Because of the robustness ofthis method, it is considered a preferred default approach.

There are a few special cases when view interpolation is not used duringa transition from image C_(j) to C_(k). First, if the cameras observe nocommon points, certain embodiments may have no basis for interpolatingthe images. In such embodiments, the system can be configured to fadeout the start image, move the camera to the destination as usual, thenfade in the destination image. Second, if the cameras are neighbors, butthe normal to the plane CommonPlane(C_(j); C_(k)) is nearlyperpendicular to the average of Direction(C_(j)) and Direction(C_(k)),the projected images would undergo a large amount of distortion duringthe morph. In this case, we revert to using a plane passing through themean of the points common to both views, whose normal is the average ofDirection(C_(j)) and Direction(C_(k)). Finally, if the vanishing line ofCommonPlane(C_(j); C_(k)) is visible in images I_(j) or I_(k), it isimpossible to project the entirety of I_(j) or I_(k) onto the plane. Inthis case, as much as possible of I_(j) and I_(k) can be projected ontothe plane, and the rest can be projected onto the plane at infinity.

Overhead Map

FIG. 3 can be used to illustrate an overhead map view. This view can beprovided in a main location in a user interface or as a side pane suchas in FIGS. 4 and 5. The overhead map view gives the user a birds-eyeview of the world. The view can be panned and zoomed in and out orrotated. The cameras may be rendered as frusta superimposed on asatellite image or map, as depicted in the sample screenshot shown inFIG. 3. This browsing mode is useful for getting a summary view of whereall the photographs in a particular data set were taken, getting a senseof where the interesting features of a particular location might be, andfor quickly finding images of particular objects. To get a clearer viewof a subset of the images, the user can use a lasso tool to select a setof images. The view will pan and zoom to focus on the selected images,and thumbnails of the images will appear on the side of the window. Theuser can also select a single image, and the browser will enter thefree-flight navigation mode, with the virtual camera initially pointedat the selected image.

Image-Based Controls

Another set of user interface features that may be provided inembodiments of the invention may be image-based in that they may be usedto guide the virtual camera from photograph to photograph. As apreprocessing step, various geometric relations can be defined on a setof photographs. In one embodiment, these relations may include oneproximity relation (is-nearby), four directional relations(is-to-the-left-of, is-to-the-right-of, is-in-front-of, is-in-back-of),and two level-of-detail relations (is-a-zoom-in-of, is-a-zoom-out-of),and are computed based on camera proximity, whether the cameras arepointed in the roughly the same direction, and the angle between thedirection two cameras are pointing and their displacement. When thevirtual camera is coincident with one of the photographs in the scene,the photographs that are related to the current photo are displayedeither as thumbnails on the border of the screen (in the case of thedirectional relations), as highlighted regions inside the currentphotograph (the is-a-zoom-in-of relation), or in a box on the side ofthe screen (the is-nearby and is a-zoom-out-of relations).

FIG. 7, for example, shows a screenshot containing a main photograph inmain location 700 surrounded by related thumbnails such as 701. Thesecontrols can be used to find photographs similar to the current photo,to see what the scene looks like from a slightly different vantagepoint, to explore a photo in more detail, or get a broader view.

Similarly, FIG. 5 illustrates a system for navigating among a pluralityof images, comprising an electronically generated interactive display(user interface) with a first digital photograph displayed at a mainlocation 500 within said display, wherein said first digital photographcomprises first image content. Here, the first image content comprisesthe street, buildings, church spires, sky and so forth. At least oneselectable alternate digital photograph is displayed at a secondlocation 503 within said display, wherein said alternate digitalphotograph comprises image content that is overlapping or proximal tosaid first image content. For example, an alternate photo in 503 maycomprise image content with a close-up view of part of the image in 500,or with a view to the left, to the right, or simply substantiallyoverlapping the photo in 500. Upon selection of a selectable alternatedigital photograph from 503, the system is configured to displaytransitional image content, as described in the “view interpolation” andsubsequent sections, in said main location 500 prior to displaying thealternate digital photograph in said main location 500. Displayingtransitional content allows for smooth, intuitive changes from a firstphoto in 500 to a second photo in 500.

When visiting a photograph C_(curr), the user has a snapshot of alocation from a single point of view and an instant in time. The usercan pan and zoom to explore the photo, but might also want to seeaspects of the scene beyond those captured in a single picture; he orshe might wonder, for instance, what lies just outside the field ofview, or to the left of the objects in the photo, or what the scenelooks like at a different time of day.

To make it easier to find related views such as these, an embodiment ofthe invention can provide the user with one or more “geometric” tools.Icons associated with these tools may appear, for example, in two rowsin the information pane 502 in FIG. 5, which pops up when the user isvisiting a photograph. A close up view of tools that may be presented in502 is provided in FIG. 9, comprising information and selectable toolspane 900. To implement these tools, an exemplary implementation may takea scene-centered, as opposed to a camera-centered, approach. In otherwords, these tools can find photos that depict parts of the scene withcertain spatial relations to what is currently in the main location 500view.

One mechanism for implementing these search tools is to project thepoints observed by a current camera, Points(C_(curr)) into other photosin an image set or database, or vice versa. A view may be selected basedon the projected motion of the points. For instance, to answer the query“show me what's to the left of this photo,” the system can be configuredto attempt to find a photo in which Points(C_(curr)) appear to havemoved right. In these “geometric” searches, new images are selected fromthe set of neighbors of the current photo. We define the neighbors of Cto be the set of cameras that observe at least one point in common withC, i.e.,Neighbors(C)={C _(j)|Points(C _(j))∩Points(C)≠0}

A first row exemplary row of tools 910 can find related images atdifferent scales. These tools select images by estimating visibility and“how large” a set of points appears in different views. Because we donot have complete knowledge of the geometry of the scene, to check thevisibility of a point in a camera we simply check whether the pointprojects inside the camera's field of view. To estimate the apparentsize of a point set in an image, we project the points into that view,find the axis-aligned bounding box of the projections, and calculate theratio of the area of the bounding box (in pixels) to the area of theimage.

A first exemplary tool 911 finds details, or higher-resolutionclose-ups, of the current photo, and is useful for quickly finding outwhich objects in the current view can be seen in more detail. Oneexemplary implementation may deem a neighbor C_(j) of C_(curr) a detailif most points in Points(C_(j)) are visible in C_(j) and the apparentsize of Points(C_(j)) in image is less than 0.75. The details may besorted by increasing apparent size and displayed in the thumbnail pane.This tool 911 thus finds alternate digital photographs comprising imagecontent with a close-up view of at least one portion of first imagecontent in a main location such as 500 in FIG. 5.

The second exemplary tool 912 finds similar photos, and is useful forcomparing similar views of an object which differ in other respects,such as time of day, season, year, and so on. For this operation, oneexemplary implementation may deem a neighbor C_(j) to be similar toC_(curr) if most points in Points(C_(curr)) re visible in C_(j), theapparent size of Points(C_(curr)) in C_(j) is within 30% of the apparentsize of Points(C_(curr)) in C_(curr) itself, and the angle between thetwo camera's viewing directions is less than a threshold. This searchtool 912 thus finds alternate digital photographs which can be placed ina location such as 503, and which may comprise image content that isoverlapping substantially all of the first image content of the image inmain location 500 without substantial additional image content. S

Such alternate photographs may be usefully sequenced according to a timewhen said alternate digital photographs were captured, if available fromthe metadata associated with the photo, e.g., from the EXIF tags. Thismay be usefully coupled with a selectable slideshow tool 940 for causingthe system to sequentially display, for example, a plurality of digitalphotographs in the main location 500, said plurality of digitalphotographs consisting substantially of the same content as the firstimage in the main location 500, and varying in a time when said digitalphotographs were captured. Slide shows and stabilized slideshows arediscussed further below.

The third exemplary tool 913 finds photographs that are “zoom-outs” ofthe current image, i.e., photos that show more surrounding context. Oneexemplary implementation may deem a neighbor C_(j) of C_(curr) to be azoom-out of C_(curr) if the apparent size of Points(C_(curr)) in I_(j)is smaller than the apparent size of Points(C_(curr)) in I_(curr)itself. The zoom-outs are sorted by increasing bounding box area. Asearch tool 913 thus finds alternate digital photographs comprisingimage content that is overlapping substantially all of the first imagecontent in a main location 500, and further comprising substantialadditional image content, thereby providing a zoomed-out view of saidfirst image content.

A second exemplary set of tools 920 may give the user a simple way to“step” left 921 or right 922, up 924, or down 925, i.e., to see more ofthe scene in a particular direction. For each camera, a “left” and“right” image may, in some embodiments, be precomputed, and displayedthem as thumbnails 921 a-925 a. To find a left and right image forcamera C_(j), embodiments may compute the average 2D flow m_(jk) of theprojections of Points(C_(j)) from image Ij to each neighboring imageI_(k). If the angle between m_(jk) and the desired direction is small(and the apparent size of Points(C_(j)) in both images is comparable),then C_(k) is a candidate left or right image. Out of all thecandidates, one embodiment may, for example, select the image I_(k)whose motion magnitude ∥m_(jk)∥ is closest to 10% of the width of imageI_(j). Tools 921 and 922 may provide, for example, at least twoselectable alternate digital photographs 921 a and 922 a, a firstalternate digital photograph 921 a comprising image content proximal toa left side of first image content in a main location 500, and a secondalternate digital photograph 922 a comprising image content proximal toa right side of such first image content.

Along with the left and right cameras, one embodiment may show a “stepback” camera, which is a shortcut to the first “zoom-out” chosen by theprocedure described above.

Interactive Tours

Additional exemplary features of a user interface implemented using thesystems and methods discussed herein may be directed to a browsing modethat supports interactive tours. This mode is similar to the free-flightnavigation mode, in that the user is guided from photograph tophotograph, but the controls may be, in some embodiments, morerestricted and the experience may have been pre-authored. This mode isanalogous to a slideshow, but the system can be configured withadditional features to provide a more interactive experience with morecontext. As in the free-flight navigation tool, a set of relations maybe created on a set of images, either automatically or manually using anauthoring tool. The user can then explore the image set using theimage-based controls described in the previous section, or may wish tobe automatically guided along a tour through the photographs.

Again, view interpolation as described above may be used to add contextto the transitions between photographs. At each photograph (each stopalong the tour), the user may pause for some time, zoom and pan aroundthe image or panorama, and may access more information about the objectsin the photograph, before moving to the next stop.

Embodiments may further support a second, object-based, type of tour.The user can choose to fix a specific object, such as the face of abuilding, in the center of the view, and view a set of photographs inwhich that object is visible. Instead of the viewpoint changing,however, the photographs are warped so as to stabilize the object ofinterest. This type of tour gives a clearer indication of how aparticular object was viewed by different people at different times. Theinteractive tour mode is especially useful in situations where aphotographer wishes to author the experience of users browsing aparticular collection of photographs. For instance, a realtor mightcreate a compelling demonstration of a home by authoring a tour of a setof photographs of the interior of the home.

In addition to the above, whenever a thumbnails pane such as 503 in FIG.5 contains more than one image, its contents can be viewed as aslideshow by pressing a play button on the pane. By default, in oneembodiment, the virtual camera may move through space from cameraposition to camera position, pausing at each image for a few secondsbefore proceeding to the next.

The user can also lock the camera, fixing it to the its currentposition, orientation, and field of view. When the images in thethumbnail pane are all taken from approximately the same location, thislocked mode stabilizes the images, making it easier to compare one imageto the next. This mode is useful for studying changes in sceneappearance as a function of time of day, season, year, weather patterns,etc. In addition to being able to view search results as a slideshow,the user can load in and view a previously saved sequence of images.This feature can be used to interactively view tours authored by otherusers.

Object-Based Browsing

Additional exemplary features of a user interface implemented using thesystems and methods discussed herein may be directed to object-basedbrowsing. The set of controls in the object-based browsing modes may beobject-based. In this mode, the user can select an object, i.e., aparticular portion of image content, in a photo by either clicking on itor dragging a box around it. Because the system has knowledge ofcorrespondence between parts of different images, embodiments may beconfigured to determine approximately where an object of interest is inother images. Therefore, based on the user's selection, the system canpresent the user with a list of thumbnails of other views of thatobject. Such embodiments may also automatically select a “good” image ofthe object of interest, based on criteria such as blurriness, lighting,and how centered and in focus the object appears.

In this regard, a search query that may be supported in an exemplarysystem is search object 930, which effectively asks the system to “showme photos of this object,” where the object in question can be directlyselected in a photograph or in the point cloud. This type of search hascertain advantages over keyword search. Being able to select an objectis especially useful when exploring a scene—when the user comes acrossan interesting object, direct selection is an intuitive way to find abetter picture of that object.

In one exemplary embodiment, the user selects the search object tool930, then selects an object by dragging a 2D rectangular box around aregion of the current photo, or around a region of the point cloud. Allpoints whose projections are inside the box are considered selected. Thesystem then takes over, searching for the “best” picture of the selectedpoints. Each image in the database can be scored based on how good arepresentation it is of the selection. The top scoring photo may beselected as the representative view, and the virtual camera may be movedto that image. The rest of the images with scores above a threshold aredisplayed in the thumbnail pane, sorted by descending score.

An example interaction is shown in FIGS. 10 and 11. FIGS. 10 and 11 areuser interfaces presented by a system for navigating among a pluralityof images. As with the other user interfaces discussed herein, they maybe accessed via an electronically generated interactive display in whicha user can move a selection pointer or the like to control the contentsof the display. In FIG. 10, an image is located within the main location1002 of a user interface display. The image in 1002 comprises imagecontent, here, the various windows and columns of a building, an archwayand a statue within the archway. The search tools 1001 may, in oneembodiment, comprise tools such as those illustrated in FIG. 9.Exemplary aspects of search tools 1001 may comprise, for example,photograph information 1010, search object 1011, zoom in, zoom out, orview photo in full size 1012, and step left, step right, and step back1013. The user may use the appropriate tool such as 1012 to select aportion of the image content in 1002, e.g. object 1000, comprising thestatute within the archway.

The system may have stored alternate images also comprising imagecontent from the photo in 1000, and can search through its variousimages for such content. In some embodiments, related photos may bepre-indexed. In other embodiments, a search may be conducted “on thefly.” Upon activation by a user of an appropriate tool, the system cansearch among said stored alternate images for a best view of a selectedportion 1000 of said image content in 1002.

In one embodiment, a best view can be defined as described above, namelyas at least one of: a view from a direction that is most closelyperpendicular to a plane associated with said portion of image content1000, a view in which a greatest number of points associated with theselected portion of said image content 1000 are visible, and a viewcomprising a greatest amount of detail associated with said portion ofimage content 1000.

In one embodiment, the system may be configured to score storedalternate images according to which have the “best view” of the selectedportion of said image content 1000. Turning to FIG. 11, the highestscoring image is placed in main location 1100, while the other imagesmay be displayed in a sequence 1102 corresponding to a score valueassociated with each displayed image. In sequence 1102, higher scoringimages may, for example, be placed to the left, while lower scoringimages are pushed to the right.

Any function that rates the “goodness” of an image with respect to aselection can be used as the scoring function. One exemplary functionmay be based on three criteria: 1) the selected points are visible, 2)the object is viewed from a good angle, and 3) the object appears insufficient detail. For each image I_(j), we compute the score as aweighted sum of three terms, E_(visible), E_(angle), and E_(detail).

When evaluating the scoring function to determine how well an imagerepresents a point select, the score for each image I_(j) can be, forexample, a weighted sum of three terms, E_(visible)+E_(angle)+E_(detail)(e.g., using {acute over (α)}= 1/3 and β=⅔).

To compute E_(visible), such an embodiment may first check whetherP_(inliers)∩ Points(C_(j)) is empty. If it is empty, then the object canbe deemed not to be visible to C_(j) at all, and E_(visible)=−∞.Otherwise,

$E_{visible} = \frac{n_{inside}}{P_{{inliers}\;}}$where n_(inside) denotes the number of points in P_(inliers) thatproject inside the boundary of image I_(j).

Next, to compute E_(angle), an exemplary embodiment may first attempt tofind a dominant plane in P_(inliers) by fitting a plane to the pointsusing orthogonal regression inside a RANSAC loop. If the plane fits mostof the points fairly tightly, i.e., if the percentage of points inP_(inliers) is above a threshold of 30%, it can be advantageous to favorcameras that view the object head-on (i.e., with the camera pointingparallel to the normal, {circumflex over (n)} to the plane), by setting:E _(angle)=Direction(C _(j))·{circumflex over (n)}.

If enough points do not fit a plane, we set E_(angle)=0.

Finally, such embodiments may compute E_(detail) to be the area, inpixels, of the bounding box of the projections of P_(inliers) into imageI_(j) (considering only points that project inside the boundary ofI_(j)). E_(detail) is normalized by the area of the largest suchbounding box of all images.

A point selection can sometimes contain points that the user did notintend to select. In particular, it may include occluded points thathappen to project inside the selection rectangle. Because the full scenegeometry is unknown, it is difficult to test for visibility. To avoidproblems due to larger-than-intended selections, one exemplaryembodiment may first prune the point set to remove likely occludedpixels. In particular, if the selection was made while visiting an imageI_(j) (and is contained in the image boundary), points can be used thatare known to be visible from that viewpoint (Points(C_(j))) to refinethe selection. Such embodiments may then compute the 3×3 covariancematrix for the selected points that are also in Points(C_(j)), andremove points with a Mahalanobis distance greater than 1.2. If theselection was made directly on the projected point cloud (i.e., not onan image) the covariance matrix may be defined with respect to theentire selected point set.

Browsing with Metadata

Exemplary systems may implement user interface features that allow usersto browse with metadata and transfer metadata among photographs in novelways. Such features may be particularly useful for large data set, e.g.with hundreds or even thousands of photographs, or for data sets thatexhibit significant variation in attributes such as time. In suchscenarios, it may be useful and informative to organize images alongother axes in addition to location. FLICKR® and other photo-sharing websites use a variety of different metadata to organize photos, includingdata, photographer, and user-specified tags. Embodiments of theinvention may integrate these metadata into the system, in addition tothe various other novel metadata and uses thereof described below. Whencombined with knowledge of location, these metadata allow for powerfulnew browsing features.

In one embodiment, as illustrated in FIG. 12, a system may be configuredto distinguish between two kinds of metadata: those that are intrinsicto the image itself, including attributes 1205-1208 like date andphotographer, and those that are created by the users of the system.Among the latter, there can be two kinds of metadata a user can attachto a photo: tags 1202-1204, which apply to the entire photo 1200, andannotations 1210-1211, which describe specific parts of a photograph1201. Each of these allows for different types of features, which may beintegrated with the other browsing modes in different ways.

First, both intrinsic attributes 1205-1208 and tags 1202-1204 can bedisplayed alongside the images 1200 themselves in the various browsingmodes. The desired metadata can be rendered as floating strings next toeach photograph 1200. The user can also choose to display onlyphotographs that meet certain criteria, such as photos taken in a rangeof dates, shot by a particular person, or described by a specific tag1202-1204 or attribute 1205-1208. When a photograph 1200 has beenselected and is centered in a view, the user can see the annotations1210-1211 that have been attached to that photograph 1200, highlightedby rectangles. In some embodiments, when the user hovers the mouse overan annotated region 1201, an annotation 1210 may appear as a tooltip.

Second, in one embodiment as illustrated in FIG. 13, as a special kindof interactive tour, a user can choose to view a set of photographs1301-1304 based on location and other metadata. For instance, a usermight wish to see all the photographs taken near a particular location,here, photos 1301-1304 of the front of the Notre Dame cathedral, inchronological order, perhaps to see how a building has changed overtime, or how a natural setting looks throughout the year.

Third, because we have knowledge of location and a sparse 3D pointcloud, a special feature of some embodiments may be to transferannotations, e.g. an annotation attached to the rose window annotatedportion from one photograph 1301, to another 1302-1304 (a kind ofdigital graffiti). For instance, when a user annotates a portion 1310 ofa photograph 1301, the set of pixels they highlight 1310 may include theprojections of some known 3D points—assuming the points are notoccluded. These points may also be visible in other images 1302-1304,and therefore we know approximately where to place the same annotationin those images. For example, an annotation may be transferred totransfer location 1320 in photo 1303.

Finally, tags and annotations such as those illustrated in FIG. 12 maycomprise links to additional information can be attached to images orparts of images. These tags can include hyperlinks to related web sites,as well as expert information from guidebooks, scanned maps of the area,and audio files.

Embodiments may thus allow users to add content to a scene in severalways. First, the user can register their own photographs to the scene atrun-time, after the initial set of photographs has been registered.Second, users can annotate regions of images, and these annotations canbe propagated to other images.

As discussed above, new photographs can registered on the fly, asfollows. First, the user may switch to a mode where an overhead map suchas that in FIG. 3 fills the view. The user may open a set of images301-303, which are displayed in the thumbnail panel 300, and drag anddrop an image 301 onto its approximate location on the map. After thedesired images 301-303 have been dropped, the system may be configuredto estimate the location, orientation, and focal length of each newphoto 301-303 by running an abbreviated version of the structure frommotion pipeline described above at a local level. First, SIFT keypointsmay be extracted and matched to the keypoints of the twenty camerasclosest to the initial location; the matches to each other camera can bepruned to contain geometrically consistent matches; the existing 3Dpoints corresponding to the matches may be identified; and finally,these matches can be used to refine the pose of the new camera. After aset of photos have been dragged onto the map, it generally takes aroundten seconds to optimize the parameters for each new camera on our testmachine, a 3.40 GHz Intel Pentium 4.

As explained above, a unique feature of embodiments of the invention isthat annotations can be automatically transferred from one image toother images that contain the same scene region(s). Returning to FIG.13, a user can select a region 1310 of an image and enter a textannotation. That annotation is then stored, along with the selectedpoints, and appears as a semi-transparent rectangular box around theselected points. Once annotated, an object can also be linked to othersources of information, such as web sites, guidebooks, and video andaudio clips. When an annotation is created, it may be automaticallytransferred to all other relevant photographs 1302-1304.

To determine if an annotation is appropriate for a given camera C_(j),embodiments may check for visibility and scale. To determine visibility,one exemplary embodiment can test that at least one of the annotatedpoints is in Points(C_(j)). To check that the annotation is at anappropriate scale for the image, the apparent size of the annotation inimage I_(j) can be determined For example, if the annotation is visibleand the apparent size is greater than 0.05 (to avoid barely visibleannotations), and less than 0.8 (to avoid annotations that take up theentire image), one embodiment may transfer the annotation to C_(j). Whenthe user visits C_(j), the annotation can be displayed as a box aroundthe projections of the annotated points, with the label in the center ofthe box.

Besides quickly enhancing a scene with semantic information, the abilityto transfer annotations has several applications. First, it enables asystem in which a tourist can take a photo (e.g., from a camera phonethat runs software according to an aspect of the invention) andinstantly see information about objects in the scene super-imposed onthe captured image. In combination with a head-mounted display, such acapability could offer a highly portable, computer-vision-basedaugmented reality system.

Second, it makes labeling photographs in preparation for keyword searchmore efficient. If an object is annotated with a set of keywords in onephoto, transferring the annotation to other photographs enables multipleimages to be added to a keyword search database based on a singleannotation. We can also leverage the many existing images that havealready been annotated. There are several sources of existingannotations. On FLICKR®, for instance, users can attach notes torectangular regions of photographs. Tools such as LabelMe encourageusers to label images on the web, and have accumulated a database ofannotations. By registering such labeled images with an existingcollection of photographs using our system, we can potentially transferthe existing labels to every other photograph in the system, therebyintegrating photo information as never before. Other images on the webare implicitly annotated. For instance, an image appearing on aWikipedia page is annotated with the URL of that page. By registeringsuch images, we can link other photographs to the same page.

Application Scenarios

In this section, we enumerate several scenarios where the techniquesdescribed herein could be used. First, various aspects of the inventioncould be used to augment a user's own collection of photographs. If thephotos were taken in a few common locations (a home, for instance),embodiments of the invention can be an effective way to organize andvisualize these photos. The user can also label parts of buildings orareas in a house, for instance, and these labels will be automaticallytransferred to other photographs. The user can also easily find photosof a particular location, or have the system choose a “best” photo ofthat location, given criteria for evaluating different photographs. Theuser can also use our tools to show their pictures to other people in away that can be more informative and engaging than a normal slideshow orphoto album.

Second, embodiments could be used to browse photos either gatheredautomatically from crawling the web or contributed by many users to acentral database. The number of photographs available online is alreadyvery large and will only grow larger as more people start postingphotographs on the Internet. Many, though not all, of these photos canbe identified by location, especially those depicting famous buildingsor natural landmarks, and are therefore amenable to the techniques wehave described. By combining photographs from many users (perhapsregistering them using a .skeleton. model created by photographs takenby an expert for this purpose), detailed image-based models can be builtof such scenes, and can be explored in compelling ways along axes suchas time and space. New users can upload their own photographs and seehow they fit in with everybody else's, can see other user's annotationson their photos, and can access additional information through links toweb pages and guidebooks.

Finally, embodiments could be used to provide real-time informationabout a location to tourists or people unfamiliar with a particularplace. If a person encounters an building or landmark of interest, weenvision that he or she could simply snap a photo of it, have the photobe automatically uploaded to a server for processing, and quicklyreceive information about where they are standing. Our system alsoallows the user's photo to be automatically annotated with additionalinformation, such as names of buildings and objects, historical notes,and other users' comments.

In addition to the specific implementations explicitly set forth herein,other aspects and implementations will be apparent to those skilled inthe art from consideration of the specification disclosed herein. It isintended that the specification and illustrated implementations beconsidered as examples only, with a true scope and spirit of thefollowing claims.

The invention claimed is:
 1. A method comprising: providing, by aserver, instructions to display a first digital photograph at a mainlocation within a display, said first digital photograph comprisingfirst image content; providing, by said server, at least one selectablealternate digital photograph displayed at a second location within saiddisplay, wherein said selectable alternate digital photograph comprisesimage content that is at least one of: overlapping to said first imagecontent or proximal to said first image content; and responsive toselection of said selectable alternate digital photograph, providinginstructions to display transitional image content in said main locationprior to displaying said selectable alternate digital photograph in saidmain location, wherein said second location within said display is alocation that is overlaid on top of the first digital photograph suchthat said image content in said selectable alternate digital photographoverlaps corresponding first image content.
 2. The method of claim 1,further comprising: providing a search tool for finding alternatedigital photographs comprising image content with a close-up view of atleast one portion of said first image content.
 3. The method of claim 1,further comprising: providing, a search tool for finding alternatedigital photographs comprising image content that is overlappingsubstantially all of said first image content without substantialadditional image content.
 4. The method of claim 1, further comprising:sequencing a plurality of alternate digital photographs according to atime when said plurality of alternate digital photographs were captured.5. The method of claim 1, further comprising: providing a search toolfor finding alternate digital photographs comprising image content thatis overlapping substantially all of said first image content and furthercomprising substantial additional image content, thereby providing azoomed-out view of said first image content.
 6. The method of claim 1,wherein said at least one selectable alternate digital photographcomprises at least two selectable alternate digital photographs, a firstalternate digital photograph comprising image content proximal to a leftside of said first image content, and a second alternate digitalphotograph comprising image content proximal to a right side of saidfirst image content.
 7. The method of claim 1, further comprising:providing a selectable slideshow tool that effectuates a sequentialdisplay of a plurality of digital photographs in said main location,said plurality of digital photographs varying in either a time when saidplurality of digital photographs were captured or a viewpoint from whichsaid plurality of digital photographs were captured.
 8. The method ofclaim 7, wherein said plurality of digital photographs are geometricallystabilized by projecting them onto a common plane in the main locationwithin said display.
 9. The method of claim 1, further comprising:providing a plurality of stored alternate images also comprising saidfirst image content; and providing a tool for searching among saidplurality of stored alternate images for a best view of a selectedportion of said first image content, wherein said best view is definedas at least one of: a view from a direction that is most closelyperpendicular to a plane associated with said image content, a view inwhich a greatest number of points associated with the selected portionof said first image content are visible, or a view comprising a greatestamount of detail associated with said first image content.
 10. Themethod of claim 1, providing a score of stored alternate imagesaccording to best view of said selected portion of said first imagecontent.
 11. The method of claim 1, further comprising: displaying aplurality of stored alternate images in a sequence corresponding to ascore value associated with each displayed image.
 12. A methodcomprising: providing, by a server, instructions to display a firstdigital photograph at a main location within a display, wherein saidfirst digital photograph comprising first image content; providing, bysaid server, at least one selectable alternate digital photographdisplayed at a second location within said display, wherein saidalternate digital photograph comprises image content that is at leastone of: overlapping to said first image content or proximal to saidfirst image content; and responsive to selection of said selectablealternate digital photograph, providing instructions to displaytransitional image content in said main location prior to displayingsaid alternate digital photograph in said main location; providing aselectable slideshow tool that effectuates a sequential display of aplurality of digital photographs in said main location, said pluralityof digital photographs varying in either a time when said plurality ofdigital photographs were captured or a viewpoint from which saidplurality of digital photographs were captured, wherein said pluralityof digital photographs are geometrically stabilized by projecting themonto a common plane in the main location within said display.
 13. Themethod of claim 12, wherein said second location within said display isa location that is overlaid on top of the first digital photograph suchthat image content in said selectable alternate digital photographoverlaps corresponding first image content.
 14. The method of claim 12,further comprising: providing a search tool for finding alternatedigital photographs comprising image content with a close-up view of atleast one portion of said first image content.
 15. The method of claim12, further comprising: providing, a search tool for finding alternatedigital photographs comprising image content that is overlappingsubstantially all of said first image content without substantialadditional image content.
 16. A method comprising: providing, by aserver, instructions to display a first digital photograph at a mainlocation within a display, wherein said first digital photographcomprising first image content; providing, by said server, at least oneselectable alternate digital photograph displayed at a second locationwithin said display, wherein said alternate digital photograph comprisesimage content that is at least one of: overlapping to said first imagecontent or proximal to said first image content; and responsive toselection of said selectable alternate digital photograph, providinginstructions to display transitional image content in said main locationprior to displaying said alternate digital photograph in said mainlocation; providing a plurality of stored alternate images alsocomprising said first image content; and providing a tool for searchingamong said stored plurality of alternate images for a best view of aselected portion of said first image content, wherein said best view isdefined as at least one of: a view from a direction that is most closelyperpendicular to a plane associated with said image content, a view inwhich a greatest number of points associated with the selected portionof said first image content are visible, or a view comprising a greatestamount of detail associated with said first image content.
 17. Themethod of claim 16, providing a score of stored alternate imagesaccording to said best view of said selected portion of said first imagecontent.
 18. The method of claim 16, further comprising: displaying theplurality of stored alternate images in a sequence corresponding to ascore value associated with each displayed image.
 19. The method ofclaim 16, further comprising: providing a search tool for findingalternate digital photographs comprising image content that isoverlapping substantially all of said first image content and furthercomprising substantial additional image content, thereby providing azoomed-out view of said first image content.
 20. The method of claim 16,wherein said at least one selectable alternate digital photographcomprises at least two selectable alternate digital photographs, a firstalternate digital photograph comprising image content proximal to a leftside of said first image content, and a second alternate digitalphotograph comprising image content proximal to a right side of saidfirst image content.